The present invention relates to an apparatus for facsimile transmission and reception.
It is known in the art to transmit, in addition to facsimile video signals corresponding to a scanned original document, identification code signals indicating the transmitting station, the time of transmission, the telephone number of the transmitting station and the like. These code signals are generally transmitted prior to transmitting the facsimile pattern so that they will be printed on the top of the facsimile reproduction and attract the attention of the operator of the receiving station.
In the prior art a separate apparatus is required in both stations to accommodate the identification codes. At the receiver, the codes must be converted into video signals in the form of scan lines for printing by the plotter of the receiving station. This requires an unnecessarily complex and expensive arrangement.
It is often desirable to underline (or overline) particular characters for emphasis. In the prior art, the only method available for this capability is to transmit different codes for characters which are to be underlined and for characters which are not to be underlined. This results in twice the number of codes which must be decoded at the receiving end and a much more complex decoder.
It is possible to convert the codes into line scan video signals at the transmitter and transmit these signals directly. The receiver, even if not provided with special circuitry can reproduce the identification characters since they are in the same format as the facsimile video signals.
However, a problem exists in such an arrangement in that the high frequency bit signals are often lost in transmission over standard telephone lines or distorted due to noise in the line. Although the receiver is provided with circuitry to minimize amplitude and phase distortion and an AGC circuit, signal loss still occurs, especially in the first part of the transmission which corresponds to the identification information.
The identification information is provided at the top of the facsimile reproduction to attract the operator's attention. If this information is illegible, it will be impossible to identify the transmitting station and it will be further impossible to contact the transmitting station to request re-transmission.
One solution to this problem is to perform an error check such as a CRC check on each transmitted scan line. However, this greatly increases the time required for transmission and the cost and complexity of the receiving apparatus.